The present invention relates to a method and apparatus for dipole acoustic well logging; and more particularly, to derived shear wave velocity logging in the presence of interfering acoustic borehole arrivals, with a dipole acoustic source.
An early and widely used technique of sonic logging measures the time required for a compressional sound wave excited by a monopole (axially symmetric) source to traverse one foot of formation. This is known as the "delta t" measurement. See, e.g., Schlumberger Limited, "The Sonic Log," Log Interpretation: Volume 1, Principles, 1972, pp. 37-41. The velocity of another type of well known wave, the shear wave, also was found to be useful, and can be obtained from the conventional monopole sonic logging tool under certain circumstances.
In conventional sonic logging, the shear wave is excited when the energy from the monopole source is mode-converted and critically refracted at the borehole wall. Unfortunately, the shear velocity is not consistently available. For example, the shear velocity cannot be obtained directly in a relatively unconsolidated formation in which the shear velocity is less than or equal to the borehole fluid velocity. Under these circumstances, energy traveling at the shear velocity cannot be critically refracted, so that no shear energy propagates along the borehole wall. Moreover, under some circumstances the shear energy is highly attenuated and obscured by the compressional arrival.
As early as 1967, J. E. White proposed a tool for logging a whole family of wave types, including the shear wave, without relying on refraction. J. E. White, "The Hula Log: A Proposed Acoustic Tool," Paper I, Transactions SPWLA, Eighth Annual Logging Symposium, 1967. The tool, which included four point force pad-mounted transducers in contact with the borehole wall, was said to be capable of exciting four borehole modes: the radial mode, the axial mode, the torsional mode, and the flexural mode. The radial and axial modes, illustrated in FIGS. 1 and 2, were considered similar to the source characteristics of conventional logging tools. The torsional mode, illustrated in FIG. 3, was said to consist entirely of shear waves. The flexural mode, illustrated in FIG. 4, was said to consist of shear waves coupled with substantial motion of the borehole wall. The vector lines in FIGS. 1-4 indicate the direction of the applied point force. Equal magnitudes are represented. Since the torsional and flexural modes of operation directly excite shear waves, shear events would be expected to be observable even in slow formations where critical refraction cannot occur. For the flexural mode, however, White found only a weak shear event from the slow shale formation.
Since White, other contact or "direct excitation" transducers have been proposed. See, e.g., U.S. Pat. No. 4,394,754, issued Jul. 19, 1983 to Waters; U.S. Pat. No. 4,380,806, issued Apr. 19, 1983 to Waters et al.; and U.S. Pat. No. 3,354,983, issued Nov. 28, 1967 to Erickson et al. Contact apparatus of this type are generally undesirable in commercial applications, because they seriously limit the speed at which the well can be logged.
White also proposed, for the flexural mode, a logging tool having a source for providing a doublet or dipole type of excitation, effected indirectly through the borehole fluid rather than directly through contact of the transducer with the borehole wall. Each of the transmitter and receiver transducers comprise split half cylinders of barium titanate. Consider the transmitter, for example; the half cylinders of the transmitter are driven by opposite polarity voltage, whereby one half cylinder emits a positive pressure pulse and the other emits a negative pressure pulse. See U.S. Pat. No. 3,593,255, issued Jul. 13, 1971 to White.
Other types of logging systems employing indirect dipole excitation sources have been proposed for shear wave logging. Kitsunezaki proposed a dipole source having a coil-driven bobbin. The movement of the bobbin ejected a volume of water in direction perpendicular to the borehole axis, while sucking an equivalent volume of water from the opposite direction. See U.S. Pat. No. 4,207,961, issued June 17, 1980 to Kitsunezaki; see also U.S. Pat. No. 4,383,591, issued May 17, 1983 to Ogura.
Other transducer designs for providing indirect excitation dipoles incorporate bender-type elements. The design described in European Patent Application Publication No. 31,989, published July 15, 1981 and naming Angona and Zemanek as coinventors, includes two disc-like piezoelectric elements bonded together and encased in a plastic potting compound. See also U.S. Pat. No. 4,383,308, issued May 10, 1983 to Caldwell.
Several theoretical studies have been conducted to understand the physics of the acoustic energy generated by the indirect excitation dipole transducer. White's analysis of a tool similar to that disclosed in U.S. Pat. No. 3,593,255, supra, assuming a source frequency of 12.5 kHz, did not suggest that a large shear event would occur; hence, White did not conclude that the method looked promising. See J. E. White, "The Hula Log: A Proposed Acoustic Tool," supra. Kitsunezaki ignored the borehole except for transducer-borehole coupling, reasoning that the wave length of the acoustic energy is sufficiently longer than the borehole diameter. See C. Kitsunezaki, "A New Method of Shear Wave Logging", Geophysics, Volume 41, Number 10, October 1980, pp. 1489-1506. He treated the wave field by approximating the environment as an infinite homogeneous solid medium, and predicted that the source pulse would propagate nondispersively in the axial direction at the shear velocity of the formation.
The flexural mode has been theoretically studied in the literature for the case of an empty borehole. See A. Bostrom & A. Burden, "Propagation of Elastic Surface Waves Along a Cylindrical Cavity and Their Excitation by a Point Force," J. Acoust. Soc. Am., Vol. 72, No. 3, September 1982, pp. 998-1004; and for a fluid-filled borehole, see W. Roever, J. Rosenbaum, & T. Vining, "Acoustic Waves from an Impulsive Source in a Fluid-Filled Borehole," J. Acoust. Soc. Am., Vol. 55, No. 6, June 1974, pp. 1144-57; R. Kumar & S. Ram, "Flexural Vibrations of a Fluid-Filled Cylindrical Cavity in an Infinite Solid Medium," Acoustica, Vol. 22, 1969-70, pp. 163-71. Yet, the flexural mode remains poorly understood, especially in contrast with the familiar tube mode.
Studies based on actual data also have been performed. The OYO Corporation in Japan has pursued Kitsunezaki's shear logging method using his dipole source. See K. Ogura, "Development of a Suspension Type S-Wave Log System," Report No. 1, OYO Technical Note TN-34, OYO Corporation, Urawa Research Institute, Japan, November 1979; K. Ogura, S. Nakanishi, and K. Morita, "Development of a Suspension Type S-Wave Log System," Report No. 2, OYO Technical Note TN-39, OYO Corporation, Urawa Research Institute, Japan, November 1980. A phenomena not predicted by Kitsunezaki's theory was noted; specifically, the frequency content of the direct shear wave was found to be strongly dependent upon the shear speed itself; formations with slow shear speeds produced a lower frequency direct shear wave than formations with faster shear speeds.
The selection of frequency characteristics of dipole sources used in well logging applications has been considered to some extent. It is known generally that the frequency of a dipole source influences the efficacy of shear wave excitation and detection. The frequency range of the transmitter transducer disclosed in the Angona et al. application is about 1 to 6 KHz, with a predominant frequency of about 3 KHz.
Despite these accomplishments, it is believed that the measurement of shear velocity using dipole and multipole sources remains imprecise under certain conditions commonly encountered in borehole logging.